Communications are very important in the modern world. Techniques for communication include public switched telephone systems (PSTNs) operating over land lines, and also include other types of terrestrial lines, such as microwave links and fiber optic buses. The increasing use of the Internet makes wideband communications more important. For many purposes, land-line communications are not suitable, as for example for communication to or with aircraft and ships at sea. For communications with such mobile stations, conventional ship-to-shore type long-wave systems are not satisfactory, in that they tend to be very narrow-band.
Communications satellites or transponders are coming into widespread use for communications with locations which are not served by land lines. Such communications systems include satellite-based mobile telephone systems. These systems include low-earth-orbit (LEO) systems, in which the satellite constellations provide low time latency because of the proximity of the communicating satellite to the earth's surface. LEO systems have not found favor, because of the complexity of the handover of signal among satellites, and because the large number of satellites required for complete coverage and system economics necessitated inexpensive, and therefore limited-capability satellites. These limited-capability satellites, in turn, have small-aperture antennas and relatively low-power transmitters. These, in turn limit the performance, and tend to make the user terminals or handsets bulky and expensive.
Geosynchronous communications satellites have the advantage of not requiring earth stations to track the spacecraft, since its position relative to a fixed location on the earth's surface remains fixed. Fewer geosynchronous spacecraft are required to provide broad coverage, but complete earth coverage is not possible, or is at least difficult. In an inclined orbit, more coverage is possible, but tracking is required. Geosynchronous orbits are, however, distant from the earth, and consequently require large-aperture antennas and relatively high transmitted power (EIRP) to provide reliable communications with earth stations and user terminals. Some geosynchronous systems, such as Echostar, are simple “bent-pipe” systems, in which the uplinked signal modulated onto a carrier is merely converted to a different carrier frequency and downlinked, with the change in frequency being for providing isolation between uplink and downlink. Other systems receive uplinked carrier signals modulated with digital signals representing independent information such as an individual mobile user conversation, and actually demodulate the independent signals to baseband and process the digital signals, as for example to regenerate the digital signals to obviate waveform degradation attributable to the uplink.
Improved communications systems are desired.